Circuit for controlling the rise of the output voltage of an amplifier at start up

Abstract: A circuit for regulating the time of charging of the output node of an amplifier at start up, commonly comprising an external soft-start capacitor charged by a current delivered by a pull-up transistor (QG19) of a push-pull output stage of the amplifier, through a decoupling diode (QG23) that is functionally connected between the output node of the amplifier and an armature of the external soft-start capacitor, is provided with a current mirror feed back circuit capable of mirroring a current function of the charge current of the external soft-start capacitor on the driving node of the pull-up transistor (QG19) of the output stage of the amplifier. The regulating circuit permits to use an external capacitance of extremely small size and upon the reaching of a fully charged condition by the external capacitor, the control circuit self-isolates and does not influence in any way the normal operation of the amplifier.

Description: The present invention relates to differential amplifiers, especially error amplifiers, used in control and regulation systems, wherein it is necessary or useful to control, at the power-on instant, the rise of the voltage on the output node of the amplifier toward the supply voltage, through the charging of capacitances associated with the output node. In control and regulation systems, differential amplifiers (error amplifiers) having an output stage composed of a pair of transistors functioning in a push-pull mode (for example a "pull-up" transistor connected between the output node and the supply rail and a "push-down" transistor connected between the output node and ground) and wherein the output node is often connected directly to the input of a comparator, are widely used. At start up, an out-of-balance condition of the inputs of the amplifier, may cause a sudden saturation rise of the output node toward the supply voltage. A steep rising of the output voltage toward the supply may cause undesired over-shoot phenomena of the value of the parameter controlled by the regulating system. Known solutions generally consist in slowing the rising of the voltage of the output node, at start up, either by introducing a circuit device for controlling the rising (slew rate) of the voltage on an internal driving node of the amplifier or on the output node itself, for example by tying the output node voltage to the voltage of a so-called "soft-start" capacitance which is charged with a constant current. In many applications, especially in case of error amplifiers, the introduction of such a device within the inner core circuit of the amplifier may present remarkable difficulties, taking into account that few millivolts may be sufficient for completely unbalancing the output stage of the amplifier. In these cases, the control of the rise time of the voltage on the output node must necessarily be effected on the output node itself without altering the core of the amplifier. A solution based on the use of a diode or a PNP transistor with an emitter connected to the output node, a collector connected to a common ground node of the amplifier and a base connected to the armature of an external "soft-start" capacitor is known. The external soft-start capacitor may be charged through a charge path that is functionally external to the structure of the amplifier. The PNP transistor prevents a too steep rise of the voltage on the output node of the amplifier. This known solution has several drawbacks and limitations. There is in fact a constant tendency to reduce as much as possible the size of the external components, in this case of the soft-start capacitor, as well as of minimizing current absorption. The use of a diode or a PNP transistor, intrinsically capable of producing a "base" current in the order of milliamperes, may often be uncompatible with a soft-start capacitor requiring a charge current of few milliamperes. In other words, the base current of the transistor may represent per se an excessive contribution to the charge current of a soft-start capaciter, which for other reasons should be as small as possible. Vis à vis these difficulties and drawbacks has now been found and represents the object of the present invention a control circuit capable of tying the output voltage of an amplifier, by employing an exernal soft-start capacitor of small size while keeping the total charge current of the exernal soft-start capacitance, in the order of microampers. Basically, the circuit of the invention implements a feedback that controls the driving level of the pull-up transistor of the output stage of the amplifier which transistor outputs the charge current of the soft-start capacitor, in order to limit the charge current delivered through a diode to the external capacitor connected to the dedicated "soft-start" pin of the integrated device, thus implementing a gradual rise of the output voltage. The feedback loop consists essentially of a complementary pair of current mirrors, through which the charge current that flows through the charge diode to the external soft-start capacitor is "mirrored", in a suitable ratio, on the base of the same transistor of the push-pull output stage of the amplifier that delivers the charge current, in order to partially reduce its drive conditions. Within the control loop, a certain current balance is achieved such that the current forced by the respective bias current generator of the output transistor that delivers the charge current of the output node, is counterbalanced, at least partially, by a current that is a function of the effective charge current, and which is fed to the base of the transistor through the current mirror of the feedback circuit of the invention. Therefore, the output node gradually charges itself toward the supply voltage. An auxiliary current generator may be employed for ensuring a complete charge of the external soft-start capacitor when, at start up, the output node of the amplifier stabilizes itself at a certain level imposed by the regulating chain of which the amplifier is part. In any case, upon the reaching of a fully charged state by the external soft-start capacitor, the charge diode (which is part of the first current mirror) cut itself off, thus isolating in practice the control soft start circuit from the functional circuit of the amplifier. The different aspects and advantages of the control circuit of the invention will become more evident through the following description of an important embodiment, herein incorporated by express reference, and by referring to the attached drawing showing the circuit diagram of an error amplifier, provided with a control circuit of the rise of the output voltage at start up, according to the present invention. The circuit of the attached figure represents a differential error amplifier, powered with a supply voltage V3 and comprising an output stage composed of a complementary pair of push-pull transistors QG19 and QG15, which have their respective collector connected in common to the output node COMP of the amplifier. The current generator MG2 provides a correct bias of the complementary pair of output transistors. As will be evident, the inner structure of the amplifier can be considered substantially irrelevant and a detailed description thereof is superfluous in the present context. The control circuit for the rising of the voltage on the output node COMP, at start up, comprises the external soft-start capacitor CSS, connected to a dedicated SS pin of the integrated circuit, a diode-configured transistor QG23, and, according to the shown embodiment of the invention, the transistors QG24, QG22, QG21, the resistances RG7, RGO/ and the transistor MG11. At start up, the capacitance CSS is completely discharged and the pull-up transistor QG19 of the output push-pull stage of the amplifier tends to deliver the maximum current (that is to saturate) to bring its collector (output node COMP) to the supply voltage V3. The current of QG19, through the diode QG23, charges the external capacitor CSS. According to a first aspect of the circuit of the invention, the current that flows through the charge diode QG23 is mirrored on QG24 and, through a second current mirror formed by the pair of transistors QG21 and QG22, complementary to the first mirror, constituted by the pair QG23 and QG24, a current proportional to the charge current that flows through the diode QG23, and suitably reduced by employing an appropriate mirror ratio, is delivered to the base of the output transistor QG19. The current delivered to the base of the output transistor QG19 partially compensates the drive conditions that otherwise would cause saturation of the transistor QG19, at start up of the circuit of the amplifier. In other words, the fractionary current mirrored on the base node of QG19 partially counterbalances the charge current forced by the respective bias generator MG2. Practically, the current balance achieved in the control loop provides for a substantial amount (I) of the total current 31, required by the bias generator MG2, to be supplied by the transistor QG21 of the second current mirror of the circuit. The control loop is capable of maintaining the total charge current of the external soft-start capacitor CSS connected to the relative SS pin, within a maximum value in the order of ten odd microampers (µA). Therefore, the circuit is capable of ensuring a gradual rise of the voltage on the output voltage COMP from about 0.7 V up to the supply voltage V3, in a strictly controlled and externally programmable mode. According to another aspect of the circuit of the invention, an auxiliary current generator MG11, capable of delivering a current of a fractionary value of the charge current level of the external soft-start capacitor CSS, may be advantageously employed for ensuring a complete charge of the CSS capacitor, also in case the output voltage COMP stabilizes on a certain value, imposed by the regulating chain of which the error amplifier may be part, at start up. When the CSS capacitor is full changed, the diode QG23 ceases to conduct, thus practically isolating the above described control circuit from the functional circuit of the amplifier. The control circuit of the invention achieves all the statedobjectives. In practice, it makes it possible to control extremely small charge currents, in the order of few ten microamperes (µA), which permits the use of external soft-start capacitors of extremely low value. On the other hand, the functional circuit of the amplifier is not altered in any way and therefore undesired interferences of the soft-start control circuit with the normal operation of the amplifier are positively excluded.

Claims: A circuit for regulating the charge of the output node (COMP) of an amplifier at start up, comprising an external capacitor that is chargeable by a current delivered by a pull-up transistor of a push-pull output stage of the amplifier, through a diode (QG23) functionally connected between the output node of the amplifier and an armature (SS) of said external capacitor, characterised by comprising means (QG21, QG22, QG24) capable of mirroring a current function of the current delivered through said charge diode to the external capacitor on the driving node of said pull-up transistor (QG19) of the output stage of the amplifier. A regulating circuit as defined in claim 1, wherein an auxiliary charge current generator is functionally connected between a supply node and said external capacitor for completing and maintaining the charge of the capacitor at the supply voltage during normal operation of the amplifier. A regulating circuit as defined in claim 1, wherein said means comprise a complementary pair of current mirrors, said charge diode forming part of a first current mirror of said complementary pair of current mirrors. A regulating circuit according to claim 2, wherein said auxiliary generator delivers a current of a fractionary value of the charge current that is delivered by said transistor of the output stage of the amplifier.

Inventors: FAGNANI MAURO, FERRARIO BRUNO, SANDRI PAOLO, FAGNANI, MAURO, FERRARIO, BRUNO, SANDRI, PAOLO
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1994
Main classification: H03F1
IPC codes: H03F3, H03F1